refactor(messagestore): extract BuildConnectionString and ApplyPragmas helpers

Pre-step for the v1.4.8 FTS5 bulk-insert worker. The worker opens its
own secondary SqliteConnection on the same db path so the WAL journal
lets parallel reads/writes through, and it has to apply the exact same
connection-string options and PRAGMAs as Connect() -- otherwise the
worker connection drifts the moment Connect grows a new pragma.

Splitting BuildConnectionString + ApplyPragmas out lets both Connect()
and the upcoming OpenSecondaryConnection() share the same source of
truth instead of duplicating the body. No behaviour change.

HellionChat/MessageStore.cs

@@ -198,22 +198,31 @@ internal class MessageStore : IDisposable
         Connection.Dispose();
     }
 
-    private SqliteConnection Connect()
+    private static string BuildConnectionString(string dbPath)
     {
         var uriBuilder = new SqliteConnectionStringBuilder
         {
-            DataSource = DbPath,
+            DataSource = dbPath,
             DefaultTimeout = 5,
             Pooling = false,
             Mode = SqliteOpenMode.ReadWriteCreate,
         };
+        return uriBuilder.ToString();
+    }
 
-        var conn = new SqliteConnection(uriBuilder.ToString());
-        conn.Open();
+    private void ApplyPragmas(SqliteConnection conn)
+    {
         conn.Execute(@"PRAGMA journal_mode=WAL;");
         conn.Execute(@"PRAGMA synchronous=NORMAL;");
         if (_platformUtil.IsWine)
             conn.Execute(@"PRAGMA cache_size = 32768;");
+    }
+
+    private SqliteConnection Connect()
+    {
+        var conn = new SqliteConnection(BuildConnectionString(DbPath));
+        conn.Open();
+        ApplyPragmas(conn);
         return conn;
     }